1. Field of the Invention
The present invention relates generally to distance measuring devices, and more particularly, to distance measuring devices for ascertaining the approximate great circle distances on a globe or certain map projections and directly displaying that distance. The present invention also relates to the measurement of electrical resistance in materials, especially thin, transparent, electrically conductive materials. The aforementioned map projections would have a minimum inherent distortion, where the distance scale is constant, and where a straight line as drawn on such a projection is equal to or approximates the great circle distance. Such map projections include, but are not limited to, Lambert Azmithul Equal Area Projections, Delesli Conic Equidistant Projections, Polar Azimuthal Projections, Lambert Conformal Conic Projections, Polyconic Projections and Oblique Cylindrical Projections. For purposes of this patent, the terms map and chart may be used interchangeably.
2. Description of the Prior Art
Devices which measure straight line distances on maps, or great circle distances on globes are known in the art. One of the most simple methods employed is the string method. A first end of a string is placed on a first point on the map or globe and a second end of the string is brought to a second point on the map or globe. The length of the string intermediate the two points is measured and this distance in compared to the scale present on the map or globe. A ruler may be effective for measuring straight line distances on a flat map; however, the ruler is defective in measuring the straight line distances on a spherical projection such as a globe, due to the inherent curvature of the globe.
Several other devices employ other methods for determining the distance between two points. U.S. Pat. No. 3,973,326 provides an example of a device for measuring distances where a movable wand is moved to a point whose distance from a datum point is to be determined. A resistor extends along the length of the wand and an electrical contact on the cursor makes point contact with a resistor. The contact and resistor form a potentiometer whose output varies with the position of the cursor. A digital voltmeter connected to the output of the potentiometer provides an indication of the distance measured. This device is employed in the analysis of seismic records. This device does not measure the electrical resistance in a thin transparent covering between two points, and also suffers from the deficiency where the device would be unable to measure distances on a spherical surface such as a globe.
Another device for electronically measuring distances is U.S. Pat. No. 4,941,267 which is employed to measure linear distance on scaled drawings. A rectangular device includes a side mounted stylus which is moved between a first and second point on the drawing. An electrical voltage signal is generated, which is then converted to frequency. A first and second binary count is generated by the circuit therein and an output of feet and inches is generated. This device does not measure the electrical resistance in a thin transparent covering between two points, and also suffers from the deficiency where the device would be unable to measure distances on a spherical surface such as a globe.
Thus, while the foregoing body of prior art indicates it to be well known to use electrical devices to measure distances on flat surfaces which employ voltage signals, the concept of covering the flat surface with a thin, transparent, electrically conductive material, and measuring the electrical resistance between two points located thereon, has not been shown. No prior art discloses the processing of a electrical resistance measurement to an output representative of the distance between the two above mentioned points as well. Nor has the concept of covering a spherical object, such as a globe, with a thin, transparent, electrically conductive material been shown, and measuring the electrical resistance between two points located thereon, also has not been shown. The simple and cost effective device of the instant invention is not contemplated. Nor does the prior art described above teach or suggest a educational device which may be used by individuals studying distances between major cities, geographical sites of interest, or other geological features on a globe. Other advantages of the present invention over the prior art also will be rendered evident.